A rimmed baking sheet is probably one of the most versatile tools in the kitchen. Not only can it be used for baking cookies but it can be a roasting pan for vegetables. With a fitted rack it can roast meats while draining unwanted grease, like here, or a mean batch of jalapeño poppers. Baking a pie? Place a rimmed baking sheet under your pie to catch dripping juices which can smoke your oven AND it will make removing your pie from the oven much easier. And probably the most elementary function of the pan would be a convenient tray for prepping food or for finished foods like when grilling outside. Using a rimmed baking sheet is safer for transporting large batches of finished meats with their flowing juices than a shallow serving plate or a paper plate for that matter!
Norpro 18"x12" Commercial Grade Aluminum Jelly Roll Pans come for a great price at $16.35 on amazon.com, here. According to ATK, you want to look for a lightly colored sheet pan for even heat and browning and you want something with some weight and heft. It's also a good idea to register for two especially since they're so cheap but also because it's handy to have two available. For example, when baking cookies it's better to have two sheets available and just one. The second pan can be prepped with the next batch of cookie dough while the first one bakes. Putting cookie dough on a hot baking sheet can cause the dough to prematurely melt and run and it might alter the finished product.
If your oven is an older model it might be a good idea to measure the inside of your oven space before purchasing an 18"x12" sheet. My home oven in CA was older and I discovered that the width was slightly smaller than 18" so I had to purchase a smaller pan. Next time: rimmed sheet pan accessories!